The present invention relates to a coherent receiver and a coherent receiving method comprising an improved channel estimation technique.
Traditionally channel estimators for example for coherent CDMA systems are based on a correlator. However, channel estimators based on this traditional approach are subject to the effects of interference, which causes performance to be degraded.
Traditionally channel estimates for coherent CDMA systems are based on a cyclic correlator (or a taped delay line). The incoming received chip stream is correlated by all of the different cyclic shifts of the correlator. The cyclic shifts which produce the n strongest correlator outputs are selected to be used by the n tap RAKE receiver. The delays corresponding to the n cyclic shifts correspond to the n estimated path delays, whereas the n chosen complex correlator outputs correspond to the amplitudes and phases of the n paths.
However, by using a cyclic correlator, the channel estimates are subject to interpath interference, which degrades the quality of the channel estimate.
From U.S. Pat. No. 5,677,930 a method and apparatus for spread spectrum channel estimation is known. Said apparatus comprises a receiver which demodulates CDMA signals using estimates of channels coefficients. Code division multiple access (CDMA) is a channel access technique which allows signals to overlap in both time and frequency. CDMA therefore is a type of spread spectrum communications. In a CDMA system, each signal is transmitted using spread spectrum techniques. In principle, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. One way to generate the signature sequence is with a pseudo-noise (PN) process that appears randomly, but can be replicated by an authorized receiver.
The informational data stream and the higher bit rate signature sequence stream are combined by multiplying the two streams together. This combination of the higher bit rate signal and the lower bit rate data stream is called spreading the information data stream signal. Each information data stream or channel is allocated a unique signature sequence.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its compliment indicates whether the information bit is +1 or xe2x88x921. The signature sequence usually comprises L bits and each bit of the signature sequence is called a chip. The entire L chip sequence or its compliment is referred to as a transmitted symbol. The conventional receiver, e.g. a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. As already set forth, the conventional RAKE receiver performs well provided several conditions or satisfied. The first condition is that the autocorrelation function of the signature sequence is ideal in that the signature sequence is uncorrelated with a shift of itself. If this is not true, then the different signal rays interfere with one another which is referred to as self-interference. For example multipath propagation destroys the orthogonality of Walsh codes often used as so called spreading codes or scrambling codes. The second condition is that the cross-correlation between the signature sequence of the desired signal and various shifted versions of the signature sequences of the other CDMA signal is 0. If this is not true, then the other CDMA signals interfere with the desired CDMA signal. This can be particularly significant when the other CDMA signal has a much higher power than the desired power CDMA signal. The third condition is that the interference caused by an echo of one transmitted symbol overlapping with the next transmitted symbol should be negligible. If this is not true, then the transmitted symbols interfere with past and future transmitted symbols, which is commonly referred to as intersymbol interference.
U.S. Pat. No. 5,677,930 thereby discloses a technique for estimating channel coefficients that eliminate the problem of self-interference in a CDMA system. According to said known technique the elimination of self-interference is accomplished by decorrelating the correlation values with respect to one another. Decorrelation is performed by multiplying an inverse of a matrix including autocorrelation function values with a vector of received bits correlated with a signature sequence associated with said autocorrelation function. As the known technique uses a matrix approach to solve the interpath interference problem, said approach is very complicated and its complexity grows exponentially with a number of multipath components.
Therefore it is the object of the present invention to provide for an improved channel estimation technique which is less complicated and less complex. For example, the complexities should only grow linearly with a number of multipath components.
Therefore it is the object to provide a channel estimation technique which eliminates the effects of interpath interference and provides better channel estimates, hence producing better receiving performance. The object of the present invention is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention.
Therefore, according to the present invention, a coherent receiver comprising a correlator for correlating a received bit stream is provided. A channel estimator estimates tap coefficients on the basis of an output signal of the correlator. The channel estimator thereby comprises interference cancellation means for eliminating interference components in the output signal of the correlator. A detector detects information symbols on the basis of the output signal of the correlator and the estimation tap coefficients. According to the present invention, the interference cancellation means comprise respectively for each estimation tap coefficient means for calculating interference components and means for subtracting interference components from an estimated tap coefficient to generate and improve an estimated coefficient to be supplied to the detector.
The means for calculating interference components can comprise cross-correlation calculation means.
The means for calculating interference components comprise scaling means for scaling the input of the cross-correlation calculation means by the number L of chips constituting one symbol of the received bit stream.
The channel estimator furthermore can comprise further cancellation means (additional cancellation stages) for eliminating interference components in the output of the first cancellation means.
The detector can comprise at least one RAKE receiver.
The interference cancellation means can be designed to cancel interpath interference components.
The channel estimator can comprise a path search means to detect the n strongest multipath component with an amplitude larger than a predetermined threshold, n being a preset integer.
According to the present invention furthermore a mobile telecommunications device comprising a coherent receiver as set forth above is provided.
According to the present invention furthermore a mobile station for a CDMA transmission system is provided comprising a coherent receiver as set forth above.
Furthermore, a coherent receiving method is provided according to which a modulated bit stream is received: The received bit stream is correlated. On the basis of an output of the correlating step tap coefficients are estimated. The tap coefficients estimating step comprises the step of interference cancellation for eliminating interference components in the output of the correlating step. Information symbols are detected on the basis of the output of the correlating step and the estimation tap coefficients. According to the present invention the interference cancellation step comprises respectively for each estimation step coefficient a step of calculating interference components and a step of subtracting interference components from an estimated tap coefficient to generate an improved estimated tap coefficient to be used in the detecting step.
The step of calculating interference components can comprise a cross-correlation calculation step and the step of scaling the input of the cross-correlation calculation step by the number L of chips constituting one symbol of the received bit stream, L being an integer.
The estimating step can further comprise the step of eliminating interference components in the output of the first cancellation means.
The detecting step can comprise at least one RAKE receiving step.
The interference cancellation step cancels interpath interference components.
The channel estimating step can comprise a path-searching step detecting the n strongest multipath components with an amplitude larger than a predetermined threshold, n being a preset integer.
The received bit stream can modulate according to a CDMA technique.